System for adjusting the anode-cathode spacing in a mercury cathode electrolytic cell, by means of single-line frames

ABSTRACT

A system for adjustment of the anode-cathode spacing in an electrolytic cell for the production of chlorine and soda comprises (FIG.  4 ) a liquid mercury electrode ( 2 ) disposed on the bottom ( 3 ) of the cell ( 100 ) and a plurality of anode electrodes ( 4 ) supported in transverse lines by mobile frames or subframes ( 11 ), adjustable in height, driven by means of respective lever systems ( 13 ) disposed between said groups of anodes ( 4 ) and an upper fixed frame ( 16 ) of the electrolytic cell ( 1 ), the lever system comprising a pair of levers ( 13 ) hinged at the fixed frame ( 16 ), the point of application of resistance thereof being connected to a respective subframe ( 11 ) and the point of application of force being connected to means of application of force ( 19, 20 ) supported by the fixed frame ( 16 ).

DESCRIPTION

[0001] The present invention relates to a system for adjusting theanode-cathode spacing in mercury cathode electrolytic cells,particularly used for the production of chlorine and soda.

[0002] The production of chlorine and soda is achieved throughelectrolysis of a saturated solution of sodium chloride in electrolyticcells of various types. Among such electrolytic cells the mercurycathode cell is of great importance.

[0003] By way of example, FIG. 1 shows a typical mercury cathodeelectrolytic cell according to the prior art, denoted as a whole byreference numeral 200.

[0004] The cell 200 comprises an iron tank 1 on the bottom 3 of whichmercury 2 flows, which forms the cathode. The bottom 3 is connected bymeans of copper bars 28 to the negative pole of a direct current powersource.

[0005] The anode of the cell 200 consists of a plurality of activatedtitanium electrodes 4, supported by one or more frames 25 that are movedmanually or by means of drive systems controlled by processors.

[0006] The electrical current is carried to the anode 4 by means ofcopper bars 5, called “current-carrying lines”, connected to thepositive pole of a direct current power source. As shown in FIGS. 2 and3, each current carrying line 5 supplies the current to a certain groupof anodes 4, in the example three anodes. Each frame 25 supports aplurality of current-carrying lines 5 and respective groups of anodes 4,in the example four current carrying lines 5 each connected to a groupof three anodes 4.

[0007] The tank (FIG. 1) of the cell is fed with brine saturated withsodium chloride which, as the current passes, decomposes and formsgaseous chlorine at the anode 4 and an amalgam of sodium and mercury atthe cathode 2. The amalgam of sodium and mercury leaves the tank 1,thanks to its sloping bottom 3, and enters a reactor 26, called adecomposer, filled with graphite and fed with water. On contact with thegraphite, the amalgam of sodium and mercury, reacts with the water andforms hydrogen, soda and mercury; the latter is returned to the tank 1of the electrolytic cell by the action of a recirculating pump 27.

[0008] The mercury 2 flowing on the bottom 3 of the tank 1 can vary inthickness either due to problems in the recirculation pump or because ofthe deposits that form on the bottom 3 during the electrolysis processon account of the impurities contained in the brine. These phenomena, inrelation to the processing conditions, can increase the thickness of themercury cathode 2 from 1 to 20 mm, and in addition can also causecontinuous variations in the anode-cathode spacing, of a local orgeneral nature, because of the undulations that are formed on thesurface of the flowing mercury.

[0009] The power consumption of the electrolytic cell 200 isproportional to the inter-electrode distance between the cathode 2 andthe anode 4. This distance is suitably maintained between 1 and 3 mm. Onaccount of the phenomena described above, however, it is difficult toadjust said inter-electrode distance and moreover there is a likelihoodof triggering short circuits between the anode 4 and the cathode 2,which destroy the electrodes 4 and create explosive situations insidethe electrolytic cell 200.

[0010] For these reasons the anodes 4 are supported by mobile framesoperated manually or by microprocessors which control the anode-cathodespacing and adjust the position of the anodes 4 according to thevariations in thickness of the mercury 2. Said control of theanode-cathode distance is indirectly achieved by monitoring both theanode-cathode voltage and the intensity of the electrical current ineach current-carrying line 5. When the anode-cathode voltage or theintensity of the current in a current-carrying line 5 exceeds the presetlimits, the frame 25 that supports that current-carrying line iscontrolled by the microprocessor to be raised or lowered in order tokeep the preset anode-cathode spacing constant.

[0011] The maximum efficiency of adjustment systems of this type isobtained by adjusting anode surfaces which are as fractional aspossible. Consequently small frames 25 are used (FIG. 4) each having thesmallest possible number of current carrying lines 5 and each currentcarrying line having the smallest possible number of anodes 4.

[0012] In practice, the optimal solution is obtained when each framesupports a group of anodes of a single current-carrying line and whenthe frame moving system is able to make movements from 0.1 to 0.2 mm. Infact it is known that voltages and power consumptions decrease more orless proportionally as the number of frames increases and as the numberof current-carrying lines and the anode surface per frame decreases. Forexample, an optimal solution is that represented by frames comprisingthe anodes of a single current-carrying line.

[0013] With reference to the electrolytic cell 200 and FIG. 2, it isobvious that it is easer to adjust/control the anode-cathode spacing bymanoeuvring eight small independent frames 25 with a surface of about 1m², instead of single frame 25 of about 8 m², especially in the presenceof the above-described variations in the thickness of the mercurycathode 2.

[0014] Almost all plants with electrolytic cells were built in the 1960sand 1970s, when the technology of automatic adjustment systems foranode-cathode spacing was not adequate and the low cost of electricpower did not justify investments to provide electrolytic cells with thenecessary technology for accurate adjustment of the anode-cathodespacing. For these reasons the electrolytic cells according to the priorart, instead of providing a suitable subdivision of the anodes, providelarge frames each comprising three to six current-carrying lines.

[0015] At present the technology provides computerized systems foradjustment and control of anode-cathode spacing which are veryeconomical. However, the existing electrolytic cells are poorly suitedto said adjustment systems because of the large size of the frames andthe resulting difficulties in movement thereof. On the other hand, thesystems for moving the frames according to the prior art are inaccurateand cannot be applied to smaller size frames.

[0016] A system for raising the frames of electrolytic cells accordingto the prior art uses four mechanical jacks disposed at the corners ofthe corresponding frames, and operated two by two by geared motors. Thissystem is valid for its precision and operation, but can be used onlyfor electrolytic cells with few, large-sized frames, in that it isextremely costly.

[0017] A lever system comprising four levers supported by four supportsat the comers of a frame is also known to the art. Each pair of leversis moved by means of a geared motor. This system also is costly anddifficult to adapt to small frames comprising one or two currentcarrying lines.

[0018] Also known to the art is a pulley system with pinion drivechains. Said system is inexpensive and lends itself to being mounted onsmall frames, but it introduces considerable play and wears easily,therefore it is inaccurate and little used.

[0019] Also known to the art is another system with a torsion bar whichcomprises a shaft that turns with angular shifting, mounted on anelectrode carrying frame. With said system adjustment of theinter-electrode distance is inaccurate, since, during operation, thelevers that permit movement of the frame change in length, introducingvertical shifts of the lever system that are not proportional to thevertical shifting of the frame. This system also is costly and can beused only for large frames.

[0020] A recently produced system, described in Italian patentapplication M197A 001434 in the name of the same applicant, achievesmovement of the frame by means of four sliders that are moved along fourinclined planes by means of two horizontal threaded shafts driven by twogeared motors. Said system is suitable for medium and small size framesand is less costly than its predecessors, but it has problems ofinertia, mechanical instability (the mobile frame can rotate along theopposite inclined planes) and of seizure both of the threaded shafts andof guides of the mobile frames. Moreover this system is not economicalfor installation on small frames comprising electrodes of a singlecurrent carrying line.

[0021] An object of the invention is to eliminate said drawbacks byproviding a system for adjustment of the anode-cathode spacing for anelectrolytic cell that is extremely precise, reliable, of simpleconstruction and inexpensive.

[0022] Another object of the present invention is to provide such asystem for adjustment of the anode-cathode spacing that can be installedon already existing frames of electrolytic cells.

[0023] Another object of the present invention is to provide such anadjustment system that is able to operate with automatic controlsystems.

[0024] These objects are achieved in accordance with the invention, asset forth in appended independent claim 1.

[0025] Other advantageous embodiments of the invention are described inthe dependent claims.

[0026] Essentially the system for adjustment of the anode-cathodespacing according to the invention comprises a plurality of small mobileframes, hereinunder referred to as subframes. Each subframe supports theanodes of a single current carrying line. Two levers fixed underneaththe pre-existing frames and supported thereby are associated with eachsubframe. The levers are operated by a geared motor that causes raisingor lowering of the subframe.

[0027] This adjustment system offers various advantages.

[0028] In fact the possibility of moving the subframes independentlyfrom each other, permits extremely precise adjustment of theanode-cathode spacing irrespective of the undulations of the mercurycathode.

[0029] Moreover the lever system for lowering and raising the subframeslends itself perfectly to the application of automatic control systemsthat can adjust anode-cathode spacing with extreme precision.

[0030] Lastly the system for adjustment of anode-cathode spacingaccording to the invention can be applied to pre-existing electrolyticcells without disturbing configuration thereof. In fact the frame of thepre-existing electrolytic cells, which acts as a fixed supporting framefor the adjustment system according to the invention, is maintained.

[0031] Further characteristics of the invention will be made clearer bythe detailed description that follows, referring to purely exemplary andtherefore non-limiting embodiments thereof, illustrated in the appendeddrawings, in which:

[0032]FIG. 1 is a diagrammatic view, in longitudinal section, of anelectrolytic cell according to the prior art;

[0033]FIG. 2 is a top plan view of the electrolytic cell in FIG. 1;

[0034]FIG. 3 is a cross sectional view taken along the plane III-III inFIG. 2;

[0035]FIG. 4 is a cross sectional view of an electrolytic cell, providedwith a system for adjustment of the anode-cathode spacing according to afirst embodiment of the invention;

[0036]FIG. 5 is a top plan view of a portion of the electrolytic cell inFIG. 3;

[0037]FIG. 6 is a front view showing in detail only the system foradjustment of the anode-cathode spacing, according the first embodimentof the invention;

[0038]FIG. 7 is a top plan view of the adjustment system of FIG. 6;

[0039]FIG. 8 is side view of the adjustment system of FIG. 6;

[0040]FIG. 9 is a diagrammatic, axonometric view showing the system foradjustment of anode-cathode spacing according the first embodiment ofthe invention;

[0041]FIG. 10 is a diagrammatic, axonometric view of a detail of FIG. 9showing a lever for movement of the subframe;

[0042]FIG. 11 is a diagrammatic, axonometric view showing the lever formovement of the subframe, according to a second embodiment of theinvention.

[0043] A first embodiment of the system for adjustment of theanode-cathode spacing of an electrolytic cell in accordance with theinvention is described with the aid of FIGS. 4-10. In this firstembodiment the same numbers are used to indicate similar orcorresponding elements to those described previously with reference toFIGS. 1-3 illustrating the electrolytic cell 200 according to the priorart.

[0044] With reference to FIG. 4, an electrolytic cell 100 comprises aniron tank 1, on the bottom 3 of which is a mercury cathode 2. Aplurality of titanium anodes 4 are arranged at a short distance frommercury cathode 2.

[0045] The electrolytic cell 100 comprises various transverse rows ofanodes 4. In the present embodiment, as shown in FIG. 4, each transverserow comprises three anodes 4. The anodes 4 are supported by respectivecopper pins 41 for carrying the current, which hereinunder forsimplicity's sake will be identified with the anodes themselves.

[0046] The pins 41 of each transverse row of anodes 4 are connected bymeans of conducting or flexible elements 6 to a copper bar 5 which, asexplained earlier, is called the current-carrying line. Eachcurrent-carrying line 5 is thus connected to three anodes 4 disposed ona transverse row.

[0047] The tank 1 is closed by a carpet 7 through which the pins 41extend around which are disposed seals 8. The carpet 7 rests on sidewalls 9 and is secured by a section bar 10 disposed on said side walls9.

[0048] The three anodes 4 of a transverse row are fixed, by means ofsteel tie rods 12 to a mobile frame 11, identified hereinunder by thename of subframe. The subframe 11 is supported by two levers 13 by meansof four tie-rods 14 (see also FIGS. 8-10), disposed in the vicinity ofthe four corners of the subframe 11.

[0049] As better shown in FIG. 10, each lever 13 is substantiallyfork-shaped, formed by a single force arm 50 and two resistance arms 51.The force arm 50 is connected to the central part of a bar 53 disposedat right angles thereto. The two resistance arms 51 are connected to thetwo ends of the bar 53 and are parallel and opposite to the force arm50.

[0050] Each tie-rod 14 is hinged to the end of a respective resistancearm 51 of the lever 13 by means of a pin 22 placed at the point ofapplication of resistance.

[0051] Returning to FIG. 4, a fixed frame 16 is bolted by means of fourbolts 24 to a main frame 25, pre-existing in all electrolytic cells ofthe prior art. The fixed frame 16 is arranged on mobile frame 11.

[0052] At a certain distance from the pins 22, the resistance arms 51 ofeach lever 13 are hinged, by means of pins 21, to two brackets 15 boltedto the fixed frame 16. The pins 21 form the points of fulcrum of thelever 13. At the end of the force arm 50, each lever 13 is hinged, bymeans of a pin 23, to a slider 17 that slides along a verticallydisposed threaded shaft 18. The pin 23 forms the point of application offorce.

[0053] In short, each lever 13 is a first class lever, in which thepoint of application of force is situated at the pin 23, at the end ofthe force arm 50; the point of application of resistance is situatedalong the axis passing through the two pins 22 at the ends of the tworesistance arms 51 and the fulcrum is situated along the axis passingthrough the two pins 21 disposed in the two resistance arms 51 at acertain distance from the pins 22.

[0054] A motor 19 with a gear unit 20 that drives the threaded shaft 18is fixed to the fixed frame 16. Operation of the motor 19 in onedirection or the other, causes a rotary movement, clockwise oranticlockwise, of the threaded shaft 18 and a consequent raising orlowering of the slider 17. The slider 17, being hinged at respectivepoints of application of force of the levers 13, causes raising orlowering of the respective force arms 50 of the levers 13; theresistance arms 51 of the two levers 13 are raised and loweredaccordingly. Since the tie-rods 14 that support the subframe 11 arehinged at the points of application of resistance of the levers 13,lowering or raising thereof occurs, causing raising or lowering of thesubframe 11 supporting the anodes 4.

[0055] The motor 19 and the gear unit 20 can be controlled by anautomatic control logic, which, on the basis of the voltages detectedbetween the anode 4 and the cathode 2 and the currents detected on thecurrent carrying line 5, controls rotation in one direction or the otherof the motor 19.

[0056]FIG. 11 shows a second embodiment of the invention, in which likeor corresponding elements to those described in the first embodiment aredenoted by the same reference numerals.

[0057] The only difference with respect to the first embodiment is thatthe two levers 13, instead of being first class levers are second classlevers. As shown in FIG. 11, the lever 13 has the same configuration asthat of the first embodiment. The lever 13 has a force arm 50 and tworesistance arms 51.

[0058] As in the first embodiment, the force arm 50 is hinged at its endto the slider 17 by means of the pin 23.

[0059] In contrast with the first embodiment the two resistance arms 51are hinged at their ends, by means of pins 21, to the brackets 15, inturn bolted to the fixed frame 16. Consequently the fulcrum of the leveris situated at the end of the resistance arms 51. The tie-rods 14, inturn integral with the subframe 11, are also hinged on the resistancearms 51, at a certain distance from the pins 21, by means of respectivepins 22. Thus the point of application of the resistance of the lever issituated on the straight line joining the two pins 21, comprised betweenthe fulcrum and the point of application of force, thus generating asecond class lever.

[0060] In both embodiments mechanical safety stops (not shown) areprovided to prevent movements of the subframe 11 beyond the minimum andmaximum distance between the electrodes 4 and 2.

[0061] Various changes and modifications of detail within the reach of aperson skilled in the art can be made to the present embodiments of theinvention, without departing from the scope of the invention set forthin the appended claims.

1. A system for adjusting of the anode-cathode spacing in anelectrolytic cell for the production of chlorine and soda, comprising aliquid mercury electrode (2) disposed on the bottom (3) of the cell(100) and a plurality of anode electrodes (4) supported in groups byrespective mobile frames or subframes (11), adjustable in height,characterized in that said mobile frames (11) are moved by means oflever systems (13) disposed between said groups of anodes (4) and anupper fixed frame (16) of the electrolytic cell (1).
 2. A system inaccordance with claim 1 , characterized in that said anode groups (4)supported by said mobile frames (11) are formed by transverse rows ofanodes of the electrolytic cell (1), respectively connected tocurrent-carrying lines (5).
 3. A system according to claim 1 or 2 ,characterized in that said lever system (13) for movement of arespective mobile frame (11) comprises a pair of levers (13), each lever(13) being hinged to said fixed frame (16), having the point ofapplication of resistance hinged to said mobile frame (11) and the pointof application of force operationally connected to means for applicationof force (19, 20).
 4. A system according to claim 3 , characterized inthat each lever (13) is fork-shaped, having a force arm (50) and tworesistance arms (51).
 5. A system according to claim 4 , characterizedin that the point of application of force is obtained by hinging eachend of the force arms (50) of the two levers (13) to a slider (17)sliding vertically on a threaded shaft (18) that is moved by said meansof application of force (19, 20).
 6. A system according to claim 5 ,characterized in that said means for application of force comprise amotor (19) and a gear unit (20) controlled by a control logic to drivesaid threaded shaft (18).
 7. A system according to any one of claims 3to 6 , characterized in that said means for application of force (19,20) are supported by said fixed frame (16).
 8. A system according to oneof claims 4 to 7 , characterized in that the fulcrum of each lever (13)and said point of application of resistance are situated on saidresistance arms (51).
 9. A system according to claim 8 , characterizedin that said mobile frame (11) is supported, in the vicinity of its fourcomers, by four tie-rods (14) hinged at the ends of the four resistancearms (51) of the pair of levers (13), so that the fulcrum of each leveris situated between the point of application of force and the point ofapplication of resistance so as to have first class levers.
 10. A systemaccording to claim 8 , characterized in that in the vicinity of the endsof the two resistance arms (51) of each lever (13), are hingedrespectively two brackets (15) integral with the fixed frame (16), sothat the point of application of resistance is situated between thepoint of application of force and the fulcrum, so as to have secondclass levers.
 11. A system according to any one of the preceding claims,characterized in that said fixed frame (16) is integral with apre-existing frame (25) in the supporting structure of the electrolyticcell (1).
 12. An electrolytic cell for production of chlorine and soda,comprising a system for adjusting the anode-cathode spacing according toany one of the preceding claims.